Construction of an opposed-piston engine cylinder is well understood. The cylinder is constituted of a liner (sometimes called a “sleeve”) retained in a cylinder tunnel formed in a cylinder block. The liner of an opposed-piston engine has an annular intake portion including a cylinder intake port near a first liner end that is longitudinally separated from an annular exhaust portion including a cylinder exhaust port near a second liner end. An intermediate portion of the liner between the intake and exhaust portions includes one or more fuel injection ports. Two opposed, counter-moving pistons are disposed in the bore of a liner with their end surfaces facing each other. At the beginning of a power stroke, the opposed pistons reach respective top center (TC) locations in the intermediate portion of the liner where they are in closest mutual proximity to one another in the cylinder. During a power stroke, the pistons move away from each other until they approach respective bottom center (BC) locations in the end portions of the liner at which they are furthest apart from each other. In a compression stroke, the pistons reverse direction and move from BC toward TC.
A circumferential clearance space between pistons and cylinder liners is provided to allow for thermal expansion. After long hours of operation carbon builds up in this clearance space, on the top land of a piston. Carbon built up on the top land of a piston moving in this space can result in increased friction and ring wear; at worst it can cause ring jacking. In conventional four-stroke, single-piston engines, carbon removal from the top land is typically performed by scraper ring hardware mounted between the top of the cylinder liner and the cylinder head. In an opposed-piston engine, the possible sites for removing carbon are limited. An opposed-piston engine does not include a cylinder head where carbon scraper devices can be located. Liner construction further reduces the possibilities. It is preferable that carbon removal not occur near the BC locations of the pistons, where the ports are located. Carbon debris near the intake port can contaminate charge air entering the bore, thereby degrading combustion. Carbon debris in the vicinity of the exhaust port can be swept into the gas stream exiting the cylinder after combustion, thereby increasing exhaust emissions. It is therefore desirable to remove carbon from the piston top lands within the liner at locations distant from the intake and exhaust ports.
Another factor that degrades engine performance throughout the operating cycle of an opposed-piston engine is related to loss of heat through the cylinder liner. Combustion occurs as fuel is injected into air compressed between the piston end surfaces when the pistons are in close mutual proximity. Loss of the heat of combustion through the liner reduces the amount of energy available to drive the pistons apart in the power stroke. By limiting this heat loss, fuel efficiency would be improved, heat rejection to coolant would be reduced, which can allow use of smaller cooling systems, and higher exhaust temperatures can be realized, which leads to lower pumping losses. It is therefore desirable to retain as much of the heat of combustion as possible within the cylinder.
An opposed-piston engine cylinder liner constructed according to the present disclosure satisfies the objective of carbon removal, thereby increasing the durability of the engine relative to opposed-pistons of the prior art. An opposed-piston liner construction according to the present disclosure satisfies the objective of heat containment, thereby allowing opposed-piston engines to operate higher heat retention than opposed-piston engines of the prior art. In some aspects, an opposed-piston liner construction according to the present disclosure satisfies both of these objectives simultaneously.